Process of coating magnesium and magnesium alloys



Patented Apr. 17, 1945 PROCESS OF COATING MAGNESIUM AND MAGNESIUM ALLOYS Frank Arthur Allen, Warmley, Bristol, and Jack Morgan, Kingswood, Bristol, England, assignors to Magnesium Elektron Limited, London, England, a British company No Drawing. Application March 12, 1941, Serial No. 383,010. In Great Britain March 21, 1940 Claims.

This invention relates to the'treatment of magnesium and magnesium alloys in the form of castings, extrusions, forgings or sheets for the purpose of resisting corrosion.

It has previously been proposed to subject magnesium to the action of wet steam at high pressures such as 20 atmospheres in an autoclave for the purpose of producing thick insulating coatings thereon in comparatively short periods, viz., 20 minutes, but no use was made of this process in the large commercial applications now available since the use of an autoclave is objectionable and it would be assumed that an eiTective coating could not be produced at normal atmospheric pressure, at any rate in reasonably short periods of treatment.

It was also suggested to form electrically insulating layers of oxides on metallic coils of wires or bands, e. g., of aluminium, by treatment in an oxidising gas mixed with steam, a fall of temperature or pressure being produced by giving a different temperature or pressure to different parts of the mixture or of the coil or of the chamber containing it. However, this suggestion relates essentially to the electrical art and was not concerned with magnesium since coils of wires or bands are not made from magnesium and are not used in the electrical art. The only metal mentioned specifically in this suggestion was aluminium, which can be made into wire coils for electrical purposes and to which the present invention does not relate. The production of coatings on aluminium involves quite diiierent problems from the production of coatings on magnesium and a process which will produce satisfactory results with aluminium cannot usually be expected to give success with magnesium.

We have now ascertained that compact thin layers having high resistance to corrosion and capable of forming a good key or base for paints, can be produced on magnesium and magnesium alloys (hereinafter referred to as the metal") by subjecting the metal to the action of steam at about normal atmospheric pressure for reasonably short periods. The pressure may be slightly higher than normal if the steam flows over the metal because of the speed of the steam but an autoclave or similar pressure vessel is not required. The metal may be placed on shelves or suspended in a suitable chamber having an inlet opening for entry of steam from a boiler, and an opening leading to a chimney. The atmosphere with which the metal is treated may consist very largely of steam. Some air or oxygen must, however, be present. Any suitable temperature, e. g.,

ternal stresses.

between about C. and 435 C. may be employed. If desired, vapours of agents known or suitable for treating magnesium alloys or for impregnating such alloys to render them non-porous may be introduced into the steam.

The metal may be subjected to the steam treatment coincidently with a heat treatment of the metal. Heat treatment as referred to in this connection is intended to mean a heating of the metal at a temperature and for a period suitable for producing an advantageous change in its structure or for relieving internal stresses. Such heat treatment may be carried out at about 360 C.-435 C., to produce a condition of solid solution, or at a temperature of about C. to effect the well known precipitation hardening, or at 250 C.-300 C. for relieving in- Any known period required for heat treatment may be used, c. g., 1-24 hours.

The steam treatment whether combined with a heat treatment or not is preferably of at least two hours duration, although fairly good results have been obtained with treatments as short as half an hour.

It is well known that heat treatment at temperatures in the region of 360-435" C. cannot be carried out in the air owing to the ease with which magnesium becomes oxidised. In consequence it has been customary to carry out the heat treatment operations in an atmosphere devoid of oxygen and water vapour, or alternatively in an atmosphere comprising a mixture of air and sulphur dioxide or furnace gases and sulphur dioxide. We have now ascertained that whereas magnesium rich alloys are excessively oxidised when the heat treatment is carried out at such temperatures in an atmosphere comprising waste gases from burnt town-gas and either containing a few per cent of free oxygen and only a very small proportion of water vapour, or containing a considerable proportion of water vapour and a few percent of free oxygen, there is no excessive oxidation if the heat treatment is carried out in such an atmosphere containing a considerable proportion of both oxygen and water vapour and that in this case a tough adherent coating is also formed on the magnesium alloy which renders the metal highly resistant to corrosion.

A further feature of the invention therefore consists in a process for the heat treatment of magnesium rich alloys in the presence of an atmosphere containing high proportions of oxygen and water vapour. The proportion of oxygen is at least 10% and preferably of the order of 15 per cent by volume and the water vapour content is at least 20% and preferably 80 per cent by volume of the latter. The atmosphere may be produced in any appropriate manner. Conveniently when town-gas is used the required composition of the hot gases is obtainable by suitable selection of the gas-air ratio obtained by appropriate regulation of the usual valves. The required atmosphere may be achieved, e. g., by choice of fuel and appropriate arrangement of furnace conditions. The proportion of water vapour in the resulting gases may however be adjusted e. g., by means of a steam inlet.

For the purpose of comparison it may be mentioned that magnesium alloy castings heat treated at 400 C. in an atmosphere produced by the incomplete combustion of fuel gas and the total removal of water vapour therefrom were not oxidised but were not protected against subsequent corrosion. Similar castings heat treated at 400 C. in combustion gases containing about 3 per cent free oxygen and 30 per cent water vapour were badly oxidised and an atmosphere containing 2. per cent free oxygen and no water vapour gave a similar result.

On the other hand, when the magnesium alloy castings were heat treated at the temperature stated in an atmosphere of waste furnace gases containing 15 per cent free oxygen and 30 per cent water vapour, no excessive oxidation occurred and the castings developed during this treatment a tough surface coating which rendered them at least as resistant to atmospheric or chemical corrosion as do the well known chromate bath treatments.

Considerable experiment has also been carried out in connection with the preparation of the metal for steam treatment. Such pretreatmenthas been carried out in acid and alkali baths of various degrees of concentration, followed by steam and other treatments, and by corrosion tests. As a result it has been ascertained that higher corrosion resistance is obtained if the pretreatment is efiected in an alkaline bath having a concentration much higher than has heretofore been proposed. For example, it has been usual to pretreat metal in aqueous solution containing about 2 per cent caustic soda. We have, however, now found that considerably improved corrosion resistance of the subequently formed coating is aiiorded if pretreatment is efl'ected in an aqueous solution containing per cent or more of caustic soda. This is illustrated by the following experiments:

Turned cylindrical sections of cast Elektron magnesium alloy AZ.91 about 2 inches long and inch diameter were used as test pieces. After treatment they were immersed in cold 3% sodium chloride solution.

Experiment No. I

This sample was treated by the well known R. A. E. chromate process (the subject of British Patent No. 353,415) including pretreatment in 2% caustic soda solution.

The appearance after approximately 100 hours immersion in 3% sodium chloride solution was as follows: about one-third of the surface was deeply pitted and the whole surface was covered thinly with corrosion products. The sample was pitted deeply on circumferential edges. Experience has shown that the corrosion resistance of the R. A. E. chromate film seems to break down on sharp edges such as this,

' Experiment No. 2

This sample was treated in boiling 4% caustic soda for 30 minutes as a pretreatment, and then treated in steam atmosphere for 2 hours. The appearance after hours immersion was as follows: there were no pits, although the surface was slightly roughened; nodules of corrosion products were also present. The sample was far less attacked than the sample of Experiment No. 1.

Experiment No. 3

This sample was treated in boiling approximately 20% caustic soda solution for 5 minutes and treated in steam at 200 C. for two and a half hours. The appearance after 100 hours immersion was as follows: with the exception of slight discoloration, the sample was completely free from any form of attack.

If desired, the metal may be treated with superheated steam, for which purpose the steam from a boiler may pass through a heating chamber on its way to a chamber in which themetal to be treated is contained. The term steam used herein and in the appended claims is intended to include steam in gaseous or invisible moisture form. The percentages of oxygen herein mentioned refer to the normal gas analysis, i. e., the percentage by volume of a gas saturated with water vapour at 20 C., and the percentages of steam and water vapour are the actual volume percentages of these in the gas at the elevated temperature referred to.

We claim:

1. A process for the treatment of magnesium and magnesium alloys in which the metal is subjected to the action of an atmosphere containing at least 15% by volume of steam and containing at least 10 per cent of uncombined oxygen in addition to that in the steam and the remainder, gases inert during the process, and at about normal atmospheric pressure at an elevated temperature and for a period long enough to produce a coating thereon offering high resistance to corrosion.

2. A process for the treatment of magnesium and magnesium alloys in which the metal is subjected to the action of an atmosphere contain-' ing at least 15% by volume of steam and containing at least 10 per cent of uncombined oxygen in addition to that in. the steam and the remainder, gases inert during the process, at about nor mal atmospheric pressure at an elevated temperature and for a period long enough to produce a coating thereon offering higher resistance to corrosion and at a temperature and for a period adapted to produce an advantageous change in the metal by heat treatment.

3. A process for the treatment of magnesium and magnesium alloys in which the metal is subjected to the action of an atmosphere produced by burning coal-gas with excess of air and adjusting the proportion of steam so that the atmosphere contains at, least 15% by volume of steam and contains at least 10 per cent of uncombined oxygen in addition to that in the steam. and at an elevated temperature and for a period long enough to produce a coating thereon oifering high resistance to corrosion.

4. A process for the treatment of magnesium and magnesium alloys in which the metal is subjected to the action of an atmosphere produced by burning coal-gas with excess of air and adjusting the proportion of steam so that the atmosphere contains at least 15% by volume of steam and contains at least 10 per cent of uncombined 5. Process for the treatment of magnesium and a magnesium alloys in which the metal is subjected to the action of an atmosphere contain- 10 ing at least 15% by volume of steam at least 10% by volume of free elemental oxygen and the remainder, gases inert during the process, at

about normal atmospheric pressure at an elevated temperature and for a period long enough to produce a coating thereon offering high resistance to corrosion.

FRAN K ARTHUR ALLEN. JACK MORGAN. 

